A subject matter of the present invention is a novel process for the preparation of a latex by emulsion (co)polymerization of ethylenically unsaturated monomers, in which the direct in-line monitoring of the (co)polymerization is carried out by Raman spectroscopy.
The latex-based compositions capable of being obtained by the emulsion (co)polymerization process, the in-line monitoring of which is carried out by Raman spectroscopy, constitute the second subject matter of the invention.
Finally, the third subject matter of the invention relates to a device for the implementation of the abovementioned process, this device comprising a reactor, at least one optical probe, a Raman spectrometer, optical fibers, a calculator and an adjusting automaton.
Latex-based compositions are used in numerous industries, in particular in those of paints, coatings, adhesives, textiles and paper coating. Among latex preparation processes, emulsion (co)polymerization is the most commonly used process.
The properties of the latex-forming (co)polymer are closely related to its overall chemical composition but also to its macromolecular characteristics, such as the microstructure of the chains, the heterogeneity of their chemical composition, their molecular mass distribution and the fraction of optionally crosslinked (co)polymer. The necessary and desired properties are, of course, different according to the field of application of the latices. By way of examples, the desired properties are, for the coating of paper, the dry pick resistance, the wet pick resistance and the wrinkle resistance of the coated papers, for the coating of carpets, including fitted carpets, the mechanical strength and the flexibility, for adhesives, the adhesiveness and the shear strength, and for paints, the wet abrasion resistance, the blocking resistance and the film-forming temperature.
It is thus important to be able to employ a process for preparing a latex which makes it possible to obtain the latter with an overall chemical composition predefined in advance but also with predefined characteristics, in order to provide for and anticipate its future use in a field of application.
For this reason, the reproducibility of the process from one operation to another (batch to batch) is a key element in guaranteeing a consistent level of performance of the latex to the user of the latter. The reproducibility of the process and the consistent quality of the latex obtained are guaranteed by following the process instructions and by identifying the critical parameters of the latter; use may be made, for this, of a methodology, such as Statistical Process Control. In order for this to be all the more effective, it is necessary to be able to have available relevant indicators of the polymerization and preferably indicators which can be measured during the process and not after the process, so as to be able to correct situations of drift. There thus exists a need to have available a technique for the in-line monitoring of the polymerization.
It is known to define the reaction parameters of a process for the preparation of a latex by (co)polymerization in intending to obtain a latex with the appropriate properties in comparison with its use: for example, by defining the temperature profiles and the addition of the reactants, such as the monomers, during the reaction. These reaction parameters thus defined generally result in a (co)polymerization rate profile which condition the characteristics of the latex obtained.
However, it is apparent that, at the industrial level, observing the process parameters (temperature, feed profile of the monomers, pressure, and the like) does not guarantee absolute reproducibility of said process and thus the production of a latex with predefined and appropriate qualities in comparison with their future use. This is because the reaction rate profile can be affected by other factors, such as the impurities present in the reactants [water, monomer(s), surfactant(s), and the like], the fluctuation in stirring rate, the surface condition of the reactant, the fluctuation in size of the particles, and the like.
Given the great economic and industrial stake with regard to the latices, there is very great advantage in having available an optimized emulsion (co)polymerization process for their preparation which guarantees to the user, above all, a consistent level of performance conferred by latices with predefined properties. Such a process must thus exhibit improved reproducibility.
To these ends, there has now been developed, and it is this which constitutes the first subject matter of the present invention, a novel optimized process for the preparation of a latex which exhibits improved reproducibility and, in addition, which is easy to implement, which has an acceptable manufacturing cost and which can be used on an industrial scale. This preparation process by emulsion (co)polymerization of at least one kind of ethylenically unsaturated monomer is carried out by continuous in situ monitoring of the (co)polymerization comprising the following stages:
(i) incident light radiation within the spectral band situated between 200 nm and 1 400 nm, and preferably between 700 nm and 1 400 nm, is emitted into the emulsion,
(ii) the light scattered by the reaction medium is picked up and transmitted to a Raman spectrometer,
(iii) the Raman spectrum, which shows the energy of the scattered light as a function of the difference in wavelength with respect to the incident light radiation, is determined,
(iv)
a) either the intensities (areas or heights) of specific lines of the spectrum:
of un(co)polymerized free monomer(s) in the reaction medium,
and of the polymer obtained;
b) or the concentrations of un(co)polymerized free monomer(s) in the reaction medium and of the polymer obtained are calculated from the Raman spectrum using quantitative spectral analytical methods, these methods preferably being multivariable chemometric methods;
(v) the process data are subsequently calculated either from the concentrations of free monomer(s) and of the polymer obtained or from the intensities (areas or heights) of specific lines of the spectrum of free monomer(s) in the reaction medium and of the polymer obtained;
(vi) these process data are compared with reference data specific to the process for the production of the latex with the predefined properties;
(vii) and the reaction parameters, such as the temperature, the pressure, the stirring of the medium and the feeding with monomers, are adjusted in order to minimize the difference between the process data measured in-line and the reference process data.
In the context of the present invention, the Raman spectrometer can be a Fourier transform Raman spectrometer or an optical dispersive Raman spectrometer. According to a particular advantageous form, the spectrometer is a Fourier transform Raman (FT-Raman) spectrometer.
One of the advantageous characteristics of the process according to the invention is in particular its continuous implementation, requiring no withdrawal and/or preparation of sample beforehand.
A second advantageous characteristic of the process according to the invention is the minimization of its sensitivity to possible local absences of homogeneity in the medium within the reactor; this being due mainly to the simultaneous determination of the intensities of lines or of the concentrations of free monomer(s) and of the polymer obtained.
A third advantageous characteristic of the invention is the complete suitability of the process according to the invention for direct in-line monitoring carried out in situ.